Alerting: Send alerts to external Alertmanager(s) (#37298)

* Alerting: Send alerts to external Alertmanager(s)

Within this PR we're adding support for registering or unregistering
sending to a set of external alertmanagers. A few of the things that are
going are:

- Introduce a new table to hold "admin" (either org or global)
  configuration we can change at runtime.
- A new periodic check that polls for this configuration and adjusts the
  "senders" accordingly.
- Introduces a new concept of "senders" that are responsible for
  shipping the alerts to the external Alertmanager(s). In a nutshell,
this is the Prometheus notifier (the one in charge of sending the alert)
mapped to a multi-tenant map.

There are a few code movements here and there but those are minor, I
tried to keep things intact as much as possible so that we could have an
easier diff.

pkg/services/ngalert/api/util.go

@@ -257,3 +257,8 @@ func ErrResp(status int, err error, msg string, args ...interface{}) *response.N
 	}
 	return response.Error(status, err.Error(), nil)
 }
+
+// accessForbiddenResp creates a response of forbidden access.
+func accessForbiddenResp() response.Response {
+	return ErrResp(http.StatusForbidden, errors.New("Permission denied"), "")
+}